Ductal carcinoma in situ (dcis) in a young man is rarely reported. Our patient, a 25-year-old man, presented with apparent symptomatic unilateral gynecomastia. He has a strong history of cancer on both the maternal and paternal sides of his family, including breast and lung (maternal) and melanoma, colon, and pancreatic (paternal). His mother tested negative for BRCA1 and BRCA2. There is no information on paternal genetic testing. The patient was treated with left subcutaneous mastectomy. Upon histologic review of the sample, concurrent gynecomastia and dcis were discovered. To date, only 4 cases of gynecomastia and dcis have been described in younger male patients. Because only 30%–50% of patients with dcis eventually develop invasive cancer in the subsequent 10–20 years, dcis prevalence in the general population may be higher than predicted. This case underscores the importance of family history in any patient presenting with a breast mass. Patients must be made aware of the risk, however small it may be, and physicians must remain cautious of cancer in young men with gynecomastia.